The present invention relates to the control of an electric generator set including an engine and an alternator. In particular, the present invention relates to the control of the excitation or field volts (or current) of the alternator within an electric generator set.
Electric generator sets (or xe2x80x9cgensetsxe2x80x9d) are widely used to provide electric power. A genset typically includes an engine coupled to an alternator, which converts the rotational energy from the engine into electrical energy. The terminal voltage of a genset is proportional to both the magnetic flux density within the alternator, and the speed of the engine. The magnetic flux density is typically determined by controlling the field current, or excitation level, of the alternator, while the speed of the engine is typically determined by an engine governor.
It is typically desirable for a genset to produce an output voltage of a known level, since many loads are designed to receive power at a given voltage level. In particular, the power grid to which gensets are often coupled is designed to maintain particular voltage levels. Because the output voltage of the alternator of a genset is determined in part by the excitation level of the alternator, it is important to be able to control this excitation level. Controlling of the excitation level typically requires feedback information concerning the output voltage of the alternator.
Conventional alternators are typically three-phase machines that output not one but three separate voltages. The output of the alternators can be in a delta format or a wye format. In the case of a wye format, the voltages output from the alternator can be understood as three individual voltages between each given phase output and a neutral or center point of the wye. In the case of a delta format, there is no similar center point, and consequently the voltages are measured only with respect to one another. Because the output voltages from alternators are AC voltages, it is typically necessary to calculate RMS or other DC-equivalent voltages based upon the AC voltages before the information can be utilized to control the excitation level of the alternator. Determining such RMS voltages requires repeated sampling of the AC output voltages of the alternator over significant periods of time, as well as a significant number of time-intensive calculations. Consequently, many conventional genset controllers only determine one RMS voltage associated with one of the three AC output voltages from the alternator, instead of three RMS voltages.
Although such single-phase genset controllers treat the single RMS voltage as a proxy for all three RMS voltages, in reality such an assumption is often incorrect. In particular, the loads placed on the three output terminals of the alternator often can differ significantly from one another, which can produce unbalanced output voltages. As a result, single-phase genset controllers sometimes provide inappropriate control signals for controlling the excitation levels of their alternators due to incomplete information regarding the overall steady-state output of the alternators. For example, if a particular alternator is designed to provide output voltages of 240 Volts (RMS) at each of its terminals but, because of a high current draw at the terminal being measured, outputs only 235 Volts at the terminal being measured, the genset controller may end up causing the other two terminals (that are not being measured) to have voltages higher than 240 Volts when it attempts to cause the voltage of the first terminal to return to 240 Volts.
Although many conventional genset controllers are designed to obtain an indication of alternator output based upon a single output voltage, some conventional genset controllers do indeed determine three RMS or other DC-equivalent voltages that are indicative of, respectively, each of the three AC output voltages of the alternator. In order to determine the three RMS voltages, however, these three-phase genset controllers typically both require a greater amount of processing power and are less responsive, i.e., provide slower control, than the single-phase genset controllers. The greater processing power requirement and slower speed of operation are due to the large number of samples that must be obtained of the output voltages and the processing required to calculate the three RMS voltages based upon these samples.
It would therefore be advantageous if a method and apparatus were developed for regulating the excitation level of an alternator which was more complete and accurate than conventional single-phase genset controllers, and in particular provided greater accuracy under conditions where the three output voltages of the alternator were unbalanced. It would further be advantageous if the method and apparatus was quicker in operation and required less processing power than conventional three-phase genset controllers.
The present inventors have discovered that a genset controller can more accurately control the armature voltage (or field current or excitation level) of the alternator of a genset by making rapid determinations of the output voltage of one phase of the alternator and, at the same time, making less rapid but more accurate determinations of the output voltage of all three phases of the alternator. The first, rapid determinations concerning the voltage of the single phase of the alternator are used to generate a first feedback signal. The second determinations concerning the voltages of all three phases of the alternator are used to generate a second feedback signal. The second feedback signal is subtracted from a target excitation level, and the difference is then provided to a proportional integral (PI) controller. The first feedback signal is subtracted from the output of the PI controller, and the difference is then provided to an additional PI controller. The output of the additional PI controller is a control signal that is then utilized to control the excitation level of the alternator.
In particular, the present invention relates to a system for providing a control signal to control an excitation level of an alternator. The system includes a first calculation element that receives first, second and third indications of first, second and third output voltages of first, second and third phases of the alternator, respectively, and calculates a first feedback signal in dependence upon the received first, second and third indications. The system additionally includes a second calculation element that receives the first indication and calculates a second feedback signal in dependence upon the received first indication. The system further includes an intermediate signal generation element that receives a target input and the first feedback signal, and in response provides an intermediate signal. The system additionally includes a control signal generation element that receives the intermediate signal and the second feedback signal, and in response provides the control signal.
The present invention further relates to a system for providing a control signal to control an excitation level of an alternator. The system includes an outer loop means for providing a first control signal component based upon a plurality of output voltage indications from the alternator, and an inner loop means for providing a second control signal component based upon at least one of the plurality of output voltage indications from the alternator. The second control signal component provided by the inner loop means is updated at a more frequent rate than the first control signal component provided by the outer loop means.
The present invention additionally relates to a method of controlling an excitation level of an alternator. The method includes receiving first, second and third indications of first, second and third output voltages of first, second and third phases of the alternator, respectively. The method further includes calculating a first feedback signal in dependence upon the received first, second and third indications, and calculating a second feedback signal in dependence upon the received first indication. The method additionally includes determining an intermediate signal in response to a target input and the first feedback signal, determining a control signal in response to the intermediate signal and the second feedback signal, and controlling the excitation level of the alternator in response to the control signal.